Pseudorandom number generator

Abstract

One embodiment of the invention includes a pseudorandom number generator (PRNG) for generating pseudorandom numbers for a radio frequency identification (RFID) tag. The PRNG includes an analog portion configured to generate a pseudorandom number seed having a value that varies based on sampling a received RF signal. A digital portion is configured to generate a pseudorandom number based on the pseudorandom number seed generated by the analog portion.

1 . A pseudorandom number generator (PRNG) for a radio frequency identification (RFID) tag, the PRNG comprising:
 an analog portion configured to generate a pseudorandom number seed having a value that varies based on sampling a received RF signal; and   a digital portion configured to generate a pseudorandom number based on the pseudorandom number seed generated by the analog portion.   
   
   
       2 . The PRNG of  claim 1 , wherein the received RF signal is an RFID interrogation signal transmitted from an RFID reader. 
   
   
       3 . The PRNG of  claim 2 , further comprising an analog signal detector that generates a first clock signal in response to the RFID interrogation signal, the first clock signal having frequency that is greater than a sampling frequency employed by the analog portion to sample the first clock signal and to generate the pseudorandom number seed. 
   
   
       4 . The PRNG of  claim 3 , wherein the analog signal detector is part of a demodulator in an analog front-end of the RFID tag, the first clock signal having a clock frequency that is commensurate with a carrier frequency of the RFID interrogation signal. 
   
   
       5 . A passive RFID tag comprising the PRNG of  claim 3 , the passive RFID tag further comprising:
 an antenna the receives the RFID interrogation signal;   a demodulator that demodulates the RFID interrogation signal and provides a demodulated RFID interrogation signal to associated logic; and   wherein the analog signal detector comprises a detector separate from the demodulator that provides the first clock signal with a frequency that is commensurate with a carrier frequency of the RFID interrogation signal.   
   
   
       6 . The PRNG of  claim 1 , wherein the analog portion comprises:
 an oscillator that provides analog sampling clock signal; and   a gate configured to sample analog values of the received RF signal according to the analog sampling clock signal provided by the oscillator to generate digital samples that define the pseudorandom number seed.   
   
   
       7 . The PRNG of  claim 6 , wherein a frequency of the analog sampling clock signal is substantially less than a frequency of the received RF signal. 
   
   
       8 . The PRNG of  claim 1 , wherein the digital portion further comprises a linear feedback shift register (LFSR) that receives a digital representation of the pseudorandom number seed and is configured to generate the pseudorandom number based on the pseudorandom number seed in response to a shift stimulus. 
   
   
       9 . The PRNG of  claim 8 , wherein the digital portion further comprises a buffer that receives at least one digital sample corresponding to analog values of the received RF signal, the buffer being configured to shift the at least one digital sample the LFSR to generate the pseudorandom number seed. 
   
   
       10 . A passive RFID tag comprising the PRNG of  claim 1 , the passive RFID tag comprising:
 an antenna the receives the received RF signal via a wireless link; and   transponder circuitry that generates a response signal that varies based on the pseudorandom number generated by the digital portion of the PRNG.   
   
   
       11 . A method of generating pseudorandom numbers within a passive radio frequency identification (RFID) tag, the method comprising:
 receiving a radio frequency (RF) signal via an antenna;   providing a first analog clock signal based on detecting analog RF signal corresponding to the received RF signal;   generating a second clock signal by an analog oscillator that is powered in response to the RF signal, the second clock signal having a frequency that is less than a frequency of the first clock signal;   sampling the first clock signal according to the second clock signal to generate a pseudorandom number seed; and   performing digital logic operations on the pseudorandom number seed to generate a pseudorandom number.   
   
   
       12 . The method of  claim 11 , wherein the RF signal comprises receiving an RFID interrogation signal transmitted from an RFID reader. 
   
   
       13 . The method of  claim 12 , further comprising demodulating the RFID interrogation signal via a demodulator of an analog front-end of the RFID tag to generate the first clock signal. 
   
   
       14 . The method of  claim 11 , wherein generating the second clock signal comprises generating the second clock signal at a frequency that is substantially less than a carrier frequency of the received RF signal, the first clock signal having a frequency that is commensurate with the carrier frequency of the received RF signal. 
   
   
       15 . The method of  claim 11 , wherein performing the digital logic operations comprises providing the pseudorandom number seed to a linear feedback shift register (LFSR) that is configured to generate the pseudorandom number from the pseudorandom number seed in response to a shift stimulus. 
   
   
       16 . The method of  claim 15 , further comprising buffering digital samples corresponding to sampled analog values of the first clock signal and serially shifting the buffered digital samples into the LFSR to provide the pseudorandom number seed. 
   
   
       17 . A radio frequency identification (RFID) tag comprising:
 means for receiving an RFID interrogation signal;   means for generating a pseudorandom number seed having a value that varies based on sampling an analog signal derived from the received RFID interrogation signal; and   means for generating a pseudorandom number by performing digital logic operations on the pseudorandom number seed.   
   
   
       18 . The RFID tag of  claim 17 , wherein the means for generating the pseudorandom number seed further comprises means for sampling the RFID interrogation signal at each cycle of a clock signal, having a frequency that is less that a carrier frequency of the RFID interrogation signal, to generate digital samples corresponding to the pseudorandom number seed. 
   
   
       19 . The RFID tag of  claim 18 , wherein the means for generating the pseudorandom number further comprises means for buffering the digital samples to shift the digital samples serially into the means for generating the pseudorandom number. 
   
   
       20 . The RFID tag of  claim 17 , wherein the means for generating the pseudorandom number seed further comprises means for detecting a carrier frequency of the RFID interrogation signal to generate the analog signal derived from the received RFID interrogation signal.